The function of a shunt reactor is generally to provide a required inductive compensation necessary for power line voltage control and stability in high-voltage transmission lines or cable systems. The prime requisites of a shunt reactor are to sustain and manage high voltage and to provide a constant inductance over a range of operating inductions. At the same time, shunt reactors are to have low profile in size and weight, low losses, low vibration and noise, and sound structural strength.
A shunt reactor generally comprises a magnetic core composed of one or more core legs, also denoted core limbs, connected by yokes which together form one or more core frames. Further, a shunt reactor is made in such manner that a coil encircles said core leg. It is also well known that shunt reactors are constructed in a manner similar to the core type power transformers in that both use high permeability, low loss grain oriented electrical steel in the yoke sections of the cores. However, they differ markedly in that shunt reactors are designed to provide constant inductance over a range of operating inductions. In conventional high-voltage shunt reactors, this is accomplished by use of a number of large air gaps in the core leg section of the core. Said core legs are being fabricated from core segments, also denoted packets, of magnetic material such as electrical steel strips. Said core segments are made of high quality radial laminated steel sheets, layered and bonded to form massive core elements. Further, said core segments are stacked and epoxy-bonded to form a core leg with high modulus of elasticity. The core legs are constructed by alternating the core segments with ceramic spacers to provide a required air gap. Said core segments are separated from each other by at least one of said core gaps and said spacers are being bonded onto said core segments with epoxy to form cylindrical core elements. Further, said spacers are typically made of a ceramic material such as steatite, which is a material with high mechanical strength, good electrical properties and a small loss factor.
Said core is accommodated in a tank comprising a tank base plate and tank walls together with a foundation supporting the tank. It is also well known that induction devices, such as shunt reactors, are immersed in cooling medium such as oil, silicone, nitrogen or fluoro-carbons.
It is a well-known problem that the magnetic core is a source of noise in electric induction devices such as transformers and reactors, and that such noise, also denoted hum, emitted from the reactor must be limited in order not to disturb the surrounding areas. Current is flowing through electrical windings surrounding the core, thus generating a magnetic field. Therefore, alternating magnetization of the core will take place, whereby the core segments cyclically expand and contract, due to the fact that ferromagnetic materials change their shape when subjected to a magnetic field, also known as the phenomena of magnetostriction, when magnetized and demagnetized by the current flowing in the reactor windings. The magnetic core thus acts as a source of 100 Hz or twice the operating frequency of the reactor vibrations and harmonics thereof. As the magnetic field through the core alternates, the core segments will expand and contract over and over again, causing vibrations. The act of magnetization by applying a voltage to the reactor produces a flux, or magnetic lines in the core. The degree of flux will determine the amount of magnetostriction, and hence the noise level. Said vibrations produce the sound waves that create the reactor's distinctive hum.
Also the previously mentioned core gaps filled with spacers, through which magnetic flux will pass by, are sources of vibrations causing noise. This is due to the fact that when said magnetic flux alternates it tends to compress/decompress the ceramic spacers, thereby causing vibrations in the core. Dynamic electromagnetic core gap forces will cause vibrations of the core which is the major source of noise. Today there are basically two ways to reduce the magnitude of the vibrations caused by the core gap forces, e.g. by reducing core gap forces or by increasing the core gap stiffness. Since the magnitude of the core gap forces is strongly dependent on the rated power of the induction device, the most efficient way to reduce the noise is to increase the stiffness of the core gaps.
In the US, the mains voltage alternates 60 times every second (60 Hz), so that the core segments expand and contract 120 times per second, producing tones at 120 Hz and its harmonics. In Europe, where the mains supply is 50 Hz, the hum is nearer 100 Hz and its harmonics.
The vibrations generated by the magnetic core together with the weight of the core and core assembly may force the rigid base structure beneath a reactor casing into vibration. The casing sidewalls might be rigidly connected to the base structure and may thereby be driven into vibration by the stiff base members and propagate noise.
In oil immersed induction devices to which the present invention relates, the magnetic core is placed in a tank, and the vibrations are propagating by the tank base and the oil to the tank walls causing noise.